The present invention relates to cardiac pacemakers, and more particularly to implantable programmable cardiac pacemakers adapted to automatically detect and respond to the occurrence of a pacemaker-mediated tachycardia (PMT). Specifically, a pacemaker incorporating the present invention minimizes the likelihood that a PMT will be sustained; and further assures that when a true PMT is detected, an appropriate PMT response is invoked, in order to protect the pacemaker patient from remaining in a prolonged PMT condition which can not be terminated without atrial pacing.
In order to efficiently perform its function of a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, the atria (A) contract prior to the ventricles (V) in accordance with a prescribed timing or synchronized relationship, hence the term "AV synchrony." These contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the mechanical contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram (ECG) and include a P-wave, representing the depolarization of the atria; the QRS-wave (sometimes referred to as an R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and masked out by the much larger QRS-wave on the ECG.)
A pacemaker is a medical device that assists the heart in maintaining a desired AV synchrony by monitoring the atria and/or ventricles for the occurrence of P-waves and/or R-waves, and by producing stimulation pulses that are delivered to an appropriate chamber of the heart to cause that chamber to depolarize, and hence contract. (Because the main function of the pacemaker is to provide such stimulation pulses, a pacemaker is frequently referred to as a "pulse generator.") If for some reason the heart is unable to maintain its natural AV synchrony, a pacemaker is utilized to monitor the heart and to provide electrical stimulation pulses when it senses the heart is not maintaining a proper AV synchrony. A VDD or VDDR-type pacemaker, for example, monitors both the right atrium and right ventricle. If it senses an atrial depolarization and ventricular depolarization within a prescribed time after the atrial depolarization, no ventricular stimulation pulse is generated. If however, it fails to sense either the atrial or ventricular depolarization within prescribed time periods, then ventricular stimulation pulses, frequently referred to as V-pulses, are generated and delivered to the ventricular chamber of the heart at an appropriate time in order to maintain the correct heart rhythm.
One of the problems that complicates the operation of a VDD or VDDR-type pacemaker, i.e., one that is capable of sensing in both chambers of the heart and pacing in the ventricular chamber, is "retrograde conduction." Retrograde conduction occurs when the depolarization of the ventricles propagates backwards into the atria, causing the atria to depolarize prematurely. This atrial depolarization is manifest by the occurrence of a P-wave, frequently referred to as a "retrograde P-wave." A retrograde P-wave appears on the ECG to be substantially the same as a natural P-wave except that it occurs much too soon after a ventricular contraction. The retrograde P-wave follows the ventricular contraction by a relatively constant period of time that is a function of the electrical conduction path through which the depolarization of the ventricles propagates backwards. (A "natural" P-wave results from the natural AV synchrony of the heart as set by the heart's natural sinus rhythm, and is hereafter referred to as a "sinus" P-wave.) See U.S. Pat. No. 4,788,980, incorporated herein by reference, for a more thorough description of retrograde conduction.
Unfortunately, many VDD and VDDR-type pacemaker sensing circuits cannot readily distinguish between a retrograde P-wave and a sinus P-wave. A significant problem thus arises because once a P-wave is sensed, the VDD or VDDR pacemaker will typically generate a V-pulse a prescribed delay thereafter, referred to herein as the "P-V delay," unless an R-wave is sensed during the P-V delay. (It is noted that much of the literature refers to the P-V delay, as that term is used herein, as the "AV delay," or AVD. Further, some pacemakers employ one delay, a P-V delay, following a P-wave, and another slightly different delay, or AV delay, following an A-pulse. For purposes of the present invention, all such delays following an atrial event, whether an A-pulse or P-wave, are referred to herein as the "P-V delay.") If the sensed P-wave is a retrograde P-wave, an R-wave will not likely occur during this relatively short P-V delay time interval because the contraction of the ventricles just occurred prior to the retrograde P-wave. Thus, at the conclusion of the P-V delay, a V-pulse is generated by the pacemaker, causing the ventricles to again contract, which contraction causes another retrograde P-wave. This retrograde P-wave, in turn, causes another V-pulse to be generated after the P-V delay, causing the cycle to repeat, resulting in a pacemaker mediated tachycardia, or PMT. (A "tachycardia" is a very rapid rhythm or rate of the heart.)
Note that during a PMT, it is the pacemaker itself that causes or "mediates" the tachycardia by tracking each P-wave caused by retrograde conduction, and providing a ventricular stimulation pulse a programmed P-V delay thereafter. The pacemaker thus provides the forward conduction path (from the atria to the ventricles) electronically by tracking each P-wave and generating a V-pulse (ventricular stimulation pulse) if no R-wave is sensed within a prescribed time thereafter (the programmed P-V delay). The reverse or backward conduction path (from the ventricles to the atria) is provided by retrograde conduction originating with the depolarization of the ventricles, which depolarization occurs as a result of the V-pulse. Thus, retrograde conduction passes the ventricular depolarization back to the atria, causing the atria to depolarize (resulting in a retrograde P-wave), and the process repeats.
Unfortunately, a PMT can be triggered by numerous events. The most common mechanism for triggering a PMT is a premature ventricular contraction, or PVC. A PVC, in turn, is not an uncommon occurrence for most mammalian hearts. A cough or a sneeze, for example, may cause a PVC. Unfortunately, for a patient having a VDD or VDDR-type pacemaker, the occurrence of a single PVC can reset the pacemaker timing in a manner that allows the pacemaker to begin tracking retrograde P-waves, causing a PMT to occur. Such PMT, if allowed to continue for more than just a few cycles, seriously impacts the ability of the heart to efficiently perform its function of a pump. What is needed, therefore, is a system or method for accurately detecting the occurrence of a true PMT, i.e., a tachycardia that is in fact mediated by the pacemaker, and quickly respond to such true PMT once detected.
In the VDDR modality, sensor drive of the ventricular rate can also initiate retrograde conduction at various sensor rates, thereby causing a PMT if the retrograde P-wave occurs outside of the post ventricular atrial refractory period (PVARP) interval. Since the VDDR mode only senses in the atrial chamber, the absence of atrial pacing will only encourage retrograde conduction and PMT's. There is, therefore, a need to protect the pacemaker patient from endlessly tracking retrograde P-waves, which may result in high ventricular maximum tracking rates and loss of AV synchrony. In addition, traditional PMT breaking algorithms may not be able to break a PMT in the VDDR modality, since there is no atrial pacing to break the cycle of retrograde conduction. This invention addresses a new PMT response (i.e., suppression) which will protect the pacemaker patient from remaining in a prolonged PMT condition which can not otherwise be terminated due to the absence of atrial pacing and a method to allow sinus P-wave tracking if sinus activity is present.